It's not likely to help solve many crimes any more than the requirement to keep a fired shell casing from handguns has.
It gives the impression that they are trying to fight crime, and expands government, and raises the price of firearms.
While the cost of making the microstamping parts isn't going to be too high in itself. The cost of tracking all those parts, who owns the guns, how they are transferred, and how to handle when a part is replaced, is going to end up adding a significant amount to the cost of the gun. The effect is especially noticeable on low priced guns.
What liberals fear most about guns is them being easily owned by the masses. That was why there was a big push against "Saturday night specials" based on the price they were sold at, not based on functionality.
They've gone after simple guns without lots of safety features. Not because those safety features significantly reduce the risks of accidents, but because they increase the cost.
They are systematically looking for ways to price guns out of the range of most Americans to afford them, and they are willing to do so slowly and incrementally.
That’s about the best argument you can make against it. But if the libs want this, then my response is that as soon as you start putting these guys in jail for long stretches of time when we catch them, we’ll consider your request. There is no point in requiring this if you’re just going to give them a slap on the wrist anyway, as is usually the case.